The present invention relates to a novel cyclic organopolysiloxane or, more particularly, a novel cyclic organopolysiloxane having at least one benzoin group bonded to the silicon atom useful as a photosensitizer in accelerating photocuring of various kinds of photocurable resin compositions or, in particular, photocurable organopolysiloxane compositions as well as a method for the synthetic preparation of such a novel compound.
As is well known, almost all kinds of resin compositions curable by the irradiation with light or ionizing radiation such as high-energy electron beams are used as admixed with a photosensitizer in order to obtain an enhanced curing velocity by the irradiation. Several of the examples of the conventional photosensitizers are benzophenone, acetophenone, anthraquinone, benzanthrone, 4-nitrobiphenyl, 4-nitrophenol, 1,4-benzoquinone, 4,4'-dimethoxybenzophenone, N,N'-tetramethyl-4,4'-diaminobenzophenone (Michler's ketone), benzoin methyl ether and the like. When the photo- or radiation-curable resin composition is an organopolysiloxane composition as is disclosed in Japanese Patent Publications Nos. 53-36515, 52-31910, 53-2911, 53-5302, 52-40334, 54-6512, 54-43017 and 55-4770, certain disadvantages are encountered in the use of the above named organic compounds as the sensitizer since these organic compounds have rather limited miscibility with the organopolysiloxane. Namely, the photo- or radiation-curable organopolysiloxane compositions can contain only an insufficient amount of a sensitizer selected from the above named organic compounds so that the curable composition can be completely cured only by the irradiation for a relatively long period of time or uniformity of curing all over the irradiated surface is hardly ensured. When such a defective organopolysiloxane composition is used, for example, as a coating material of a peelable release paper, the sensitizer sometimes precipitates in the coating layer due to the poor miscibility with the organopolysiloxane disturbing the adhesion of the cured film to the substrate surface and the release paper cannot be imparted with satisfactory releasability.
With an object to overcome the above mentioned difficulties, several types of organosilicon compounds have been proposed as a sensitizer for photo- or radiation-curable organopolysiloxane compositions such as benzoin trialkylsilyl ethers (see Japanese Patent Publication No. 50-1597), silyl benzophenones (see Japanese Patent Publication No. 51-48794), benzoin group-containing organopolysiloxanes having a linear molecular chain structure (see Japanese Patent Kokai No. 54-44643) and the like.
These organosilicon compounds are still unsatisfactory as a sensitizer since the compounds of the former two classes have no sufficiently high miscibility with the organopolysiloxane compositions while, in the benzoin group-containing linear organopolysiloxanes, sufficient miscibility is obtained when the polysiloxane has a relatively large degree of polymerization but the content of the benzoin groups in such a high molecular polysiloxane cannot be so high relative to the other organic groups that a large amount thereof should be admixed in the photo- or radiation-curable composition leading to unavoidable decrease of the residual adhesion of the peelable release paper prepared by use of such a silicone composition.
Accordingly, there has been desired to develop a sensitizer for photo- or radiation-curable organopolysiloxane compositions having sufficient miscibility therewith and exhibiting satisfactory sensitizing effect even in a relatively small amount of addition.